Search for decays of stopped long-lived particles produced in proton-proton collisions at sqrt(s) = 8 TeV

1 MB·English

by  CMS Collaboration

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)
CERN-PH-EP/2013-037
2015/04/15
CMS-EXO-12-036
Search for decays of stopped long-lived particles produced
√
in proton-proton collisions at s = 8TeV
5
1
∗
The CMS Collaboration
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2

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p
A

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Abstract
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x
A search has been performed for long-lived particles that could have come to rest
e
- withintheCMSdetector,usingthetimeintervalsbetweenLHCbeamcrossings. The
p
existence of such particles could be deduced from observation of their decays via
e
h energy deposits in the CMS calorimeter appearing at times that are well separated
[
from any proton-proton collisions. Using a data set corresponding to an integrated

2 luminosity of 18.6fb−1 of 8TeV proton-proton collisions, and a search interval corre-
v
spondingto281hoursoftriggerlivetime,10eventsareobserved,withabackground
3
0 prediction of 13.2+3.6 events. Limits are presented at 95% confidence level on gluino
−2.5
6 andtopsquarkproduction, forover13ordersofmagnitudeinthemeanproperlife-
5
time of the stopped particle. Assuming a cloud model of R-hadron interactions, a
0
(cid:46) (cid:46)
. gluinowithmass 1000GeVandatopsquarkwithmass 525GeVareexcluded,for
1
0 lifetimesbetween1µsand1000s. Theseresultsarethemoststringentconstraintson
5 stoppedparticlestodate.
1
:
v
i
X PublishedintheEuropeanPhysicalJournalCasdoi:10.1140/epjc/s10052-015-3367-z.
r
a
(cid:13)c 2015CERNforthebenefitoftheCMSCollaboration.CC-BY-3.0license
∗SeeAppendixAforthelistofcollaborationmembers

1
1 Introduction
Many extensions of the standard model (SM) predict the existence of new heavy long-lived
particles [1–6]. At the CERN LHC the two general-purpose detectors, ATLAS and CMS, have
already set stringent limits on the existence of such particles with searches that exploit the
anomalously large ionization and/or long time-of-flight that they would exhibit as they tra-
versethedetectors[7,8]. Thesesearchesarecomplementedbythosethattargetthefractionof
suchparticlesproducedwithsufficientlylowkineticenergy(KE)that they cometorestinthe
detectors. In this approach, the subsequent decay is directly observed, allowing (in principle)
thereconstructionofthe“stopped”particleandthestudyofitscharacteristics[9].
New long-lived heavy particles, such as heavy gluinos (g(cid:101)) and top squarks ((cid:101)t), could be pair-
producedinproton-proton(pp)collisionsandcombinewithSMparticlestoformR-hadrons[10–
12]. These R-hadrons would then traverse the volume of the detector, interacting with detec-
tor materials via nuclear interactions and, if charged, by ionization. Below a critical velocity
(cid:46)
0.45c, the KE of the R-hadron is small enough and the energy loss per unit length (dE/dx)
large enough that the particle can come to rest within the body of the detector. At some later
time, the stopped R-hadron would then decay. Assuming at least one daughter particle is a
SM particle and the R-hadron has stopped in an instrumented region of the detector, the de-
cay could be observable. If this stopping location is in the calorimeter, as is most likely given
itsdensity, theexperimentalsignaturewouldbearandomly-timed, relativelylargeenergyre-
sponse spread over a few channels. Since these depositions might be difficult to differentiate
from those of SM particles produced in pp collisions, they would be most easily observed at
times between pp collisions. During these times the detector should be quiet with the excep-
tion of cosmic rays, some beam-related backgrounds, and instrumental noise. The results of
suchsearcheshavepreviouslybeenreportedbytheD0collaborationattheTevatron[13],and
bytheCMS[14,15]andATLAScollaborations[16,17].
This paper provides an update to the CMS search for stopped particles. The new analysis
benefits from a four-fold integrated luminosity increase and uses data resulting from higher
energyppcollisionscomparedtothepreviousCMSpublication[14].
2 The CMS detector and jet reconstruction
The central feature of the CMS apparatus is a superconducting solenoid of 6m internal diam-
eter, providing a magnetic field of 3.8T. Within the superconducting solenoid volume are a
siliconpixelandstriptracker,aleadtungstatecrystalelectromagneticcalorimeter(ECAL),and
a brass/scintillator hadron calorimeter (HCAL), each composed of a central (barrel) and two
forward (endcap) sections. In the region |η| < 1.74, the HCAL cells have widths of 0.087 in
pseudorapidity and 0.087 in azimuth (φ). In the η-φ plane, and for |η| < 1.48, the HCAL cells
map onto 5×5 ECAL crystals arrays to form calorimeter towers projecting radially outwards
from close to the nominal interaction point. At larger values of |η|, the size of the towers in-
creases and the matching ECAL arrays contain fewer crystals. Within each tower, the energy
deposits in ECAL and HCAL cells are summed to define the calorimeter tower energies, sub-
sequently used to provide the energies and directions of hadronic jets. Muons are measured
in gas-ionization detectors embedded in the steel flux-return yoke outside the solenoid; drift
tubes and resistive-plate chambers (RPC) provide coverage in the barrel, while cathode strip
chambers (CSC) and RPC provide coverage in the endcaps. Extensive forward calorimetry
complementsthecoverageprovidedbythebarrelandendcapdetectors. Thefirstlevel(L1)of
theCMStriggersystem,composedofcustomhardwareprocessors,usesinformationfromthe
2 3 DatasetandMonteCarlosimulationsamples
calorimeters and muon detectors to select the most interesting events in a fixed time interval
of less than 4µs. The high-level trigger (HLT) processor farm further decreases the event rate
from around 100kHz to around 400Hz, before data storage. A more detailed description of
the CMS detector, together with a definition of the coordinate system used and the relevant
kinematicvariables,canbefoundinRef.[18].
Because a stopped particle is by definition at rest, the energy deposits in the calorimeter that
result from its decay would not generally be oriented in towers radially towards the pp inter-
action point of CMS. Nevertheless, such depositions are sufficiently jet-like that they may be
reconstructed offline using the anti-k clustering algorithm [19, 20] with a distance parameter
T
of0.5.
3 Data set and Monte Carlo simulation samples
√
The search is performed using s = 8TeV pp collision data collected between May and De-
cember2012,correspondingtoanintegratedluminosityof18.6fb−1andto281hourswhenthe
dedicated trigger used in this analysis was active (“livetime”). The maximum instantaneous
luminosityachievedduringthisperiodwas7.5×1033cm−2s−1. Asacontrolsample,thisanal-
√
ysis uses s = 7TeV pp collision data corresponding to an integrated luminosity of 3.6pb−1
collectedatthebeginningofLHCoperationsin2010. Thecontrolsampleincludes253hoursof
triggerlivetime. Thoughtheintegratedluminosityforthisperiodismuchsmallerthanthatin
2012, thetriggerlivetimeiscomparablebecauseofthelongertimeintervalbetweencollisions
in2010.
Simulated signal Monte Carlo (MC) events for this analysis are generated in three stages. In
thefirststageweuse PYTHIA 8.153[21]togeneratepp → g(cid:101)g(cid:101)andpp →(cid:101)t(cid:101)tevents. Thecolored
sparticles are hadronized with the default parameters in the RHADRONS package included in
PYTHIA 8.153. These parameters influence technical aspects of the hadronization process, e.g.
the fraction of produced R-hadrons that contain a gluino and a valence gluon, which is set to
10%. Because of the nature of the stopped-particle technique, these parameters do not have a
significant effect on the phenomenology. The passage of the R-hadrons through the detector
is simulated with GEANT4 9.4.p03 [22]. A phase-space driven “cloud model” of R-hadron
interactions with the material of the CMS detector [23, 24], referred to as the “generic” model
in Ref. [16], is used to simulate the interaction of these R-hadrons with the CMS detector. In
this model, which has emerged as the standard benchmark for these searches, R-hadrons are
treatedassupersymmetricparticlessurroundedbyacloudoflooselyboundquarksorgluons.
TheKEofthesesimulatedR-hadronsisdiminishedthoughnuclearinteractionsandionization,
andtheycanthuscometorestwithinthebodyoftheCMSdetector. Ifthisoccurs,theposition
in the CMS coordinate system of the stopped R-hadron is recorded. The second stage of the
simulation generates an R-hadron, translates it to the stopping position recorded in the first
stage, and causes it to decay at rest via a second GEANT4 step. The gluino decay is simulated
as g(cid:101) → gχ(cid:101)0 and the top squark decay is simulated as(cid:101)t → tχ(cid:101)0, where χ(cid:101)0 is the lightest neu-
tralinoinbothinstances. Thefirststageallowsestimationofthestoppingprobability ε ,
stopping
and the second stage allows estimation of the reconstruction efficiency ε . While any spin
reco
correlation of the decaying gluino with its pair produced partner is lost in this approach, no
observableeffectsofthisomissionareexpected[25]. Theprobabilityforthesubsequentdecay
ofastoppedparticletooccuratatimewhenthetriggerisliveisestimatedwithacustomthird
stagepseudo-experimentMCsimulation. Werandomlygeneratedecaytimesaccordingtothe
exponentialdistributionexpectedforagivenlifetimehypothesisandcomparethesetothede-
3
livered luminosity profile and actual bunch structure of each LHC fill, and all relevant CMS
triggerrules,inordertodetermineaneffectiveluminosity(L )foreachlifetimehypothesis.
eff
4 Event selection
We perform the search with a dedicated trigger used to select events out-of-time with respect
to collisions. A series of offline selection criteria are then applied to exclude events likely due
tobackgroundprocesses.
The LHC beams are composed of circulating bunches of protons. At the experiments, bunch
crossings(BX)nominallyoccuratintervalsof25ns. In2012,however,theLHCoperatedwitha
50nsminimumintervalbetweenprotonbunchesandthemostoftenusedLHCfillingscheme
had1377ofsuchintervalscontainingcollidingbunchesofprotons. Thissearchlooksforevents
inasuitablesubsetoftheother802BXsi.e. duringintervalsthatare“out-of-time”withrespect
to normal collisions. Such events are recorded using an updated version of the calorimeter
triggeremployedintheearlierCMSstudy[14,15]thatusesthetwobeampositionandtiming
monitors (BPTX) that are positioned along the beam axis, at either end of the CMS detector
closetothebeams. TheseBPTXaresensitiveelectrostaticinstrumentsthatareabletodetectthe
passage of an LHC proton bunch. Consequently, in order to search for out-of-time events, we
employ a dedicated trigger that requires an energy deposit in the calorimeter trigger together
withtheconditionthatneitherBPTXdetectsabunchinthatBX.Wealsorequirethatatmostone
BPTX produces a signal in a window ±1 BX around the triggered event. This rejects triggers
due to out of time “satellite” bunches that occasionally accompany the colliding protons. The
energy deposition in the calorimeter of a jet from the R-hadron decay is sufficiently similar to
thoseofjetsoriginatingdirectlyfromppcollisionsthatacalorimeterjettriggercanbeused. At
L1thejettransverseenergy,whichiscalculatedassumingthejetwasproducedatthenominal
interactionpoint,isrequiredtobegreaterthan32GeV,whileintheHLTjetenergyisrequired
tobegreaterthan50GeV. AtbothL1andHLT |η | isrequiredtobelessthan3.0. Finally,the
jet
trigger rejects any event within a ±1 BX window that is identified as beam halo at L1 by the
presence of a pair of CSC hits geometrically consistent with the expected trajectory of a halo
muon.
Weselecteventsmorethan ±1BXawayfromanin-timeppcollision. Additionally,toremove
rare events in which an out-of-time pp collision occurred, usually caused by residual protons
foundinbetweenprotonbunches,wevetoeventsthatincludeaprimaryvertexreconstructed
by an adaptive vertex fit [26] with greater than or equal to 4 degrees of freedom as expected
from a pp collision. Finally, we require that a jet is reconstructed with an energy of at least
70GeV. Thisthresholdissetjustabovetheturn-onplateauforthe50GeVtrigger.
Halomuonsareasourceofbackgroundforthisanalysis. Halomuonsareproducedwhenoff-
orbitprotonsintheLHCbeamstrikematerialinsomelimitingapertureoftheLHCupstreamof
theCMSdetector. Theresultingcollisionproducesashowerofparticles,mostofwhichdecay
beforereachingCMS.Muonsareproducedinthesedecaysand,giventheirlonglifetimesand
the fact that they undergo only electroweak interactions, can survive long enough to traverse
the detector. When they pass through the denser regions, they can emit a bremsstrahlung
photon that strikes the calorimeter and can be reconstructed with large enough energy to be
includedinthesearchsample. Weremovetheseeventsbyvetoinganyeventinwhichthereare
hitsrecordedwithintheCSCforwardmuonchambers. Thelow-noiserateoftheCSCdetectors
allowstherequirementtobesetatthesingle-hitlevel,whichenablesthemaximalexclusionof
thisbackground.
4 5 Signalefficiency
MuonsfromcosmicraysincidentontheCMSdetectorcanalsomimicthesignalcharacteristics.
Similar to the halo background, cosmic ray muons may emit a photon that strikes the calori-
meter, leaving a large energy deposit. To remove such events, we consider the distribution of
reconstructed hitswithin the barrel muon system. Compared to the expectedsignal, there are
keydifferenceswithcosmicraymuonsthatcanbeexploited. ItispossiblethatheavyR-hadron
decayproductshavealargeenoughenergyto“punchthrough”theouterregionofthecalori-
meterandthefirstlayersoftheironyokeofthesolenoid,leavingenergydepositsinthemuon
system. This phenomenon is easily distinguished from cosmic ray muons by considering the
distribution of reconstructed hits. In the case of cosmic ray muons, we expect hits evenly dis-
tributed throughout the barrel of the muon system, whereas for signal events, we expect the
hits to be restricted to the innermost layers of the muon system. Additionally, in the case of
punch-through, the muon hits should be localized near the reconstructed jet. Unlike signal
events,hitsfromcosmicraymuonsmayalsoappearoppositein φ tothatofthereconstructed
jet. Exploitingtheseproperties,weareabletosubstantiallyreducethecosmicraybackground
contaminating the signal region by removing events with hits in the outer layers of the muon
system(r > 560cm),eventswithhitsrecordedinboththetopandbottomofthemuonsystem,
andeventswithhitsinthemuonsystemseparatedfromtheleadingjetinφbygreaterthan1.0
radians.
The final source of background stems from instrumental noise in the calorimetry system, pri-
marilywithintheHCAL.NoiseintheHCALcangiverisetoeventsinwhichanerrantspikein
energyisrecorded,unrelatedtoanyphysicalinteractionwithparticlesproducedinsidethede-
tector. Theseoccurrencesarerare,butthecalorimeterresponseresemblestheanticipatedsignal
and must be removed. The HCAL electronics has a well-defined time response to charge de-
positsgeneratedbyshoweringparticles. Analogsignalpulsesproducedbytheelectronicsare
sampled at 40 MHz, synchronized with the LHC clock. These pulses are readout over ten BX
samplescenteredaroundthepulsemaximum. Aphysicalsignalexhibitsaclearpeakfollowed
byanexponentialdecayoverthenextfewBXs. Moreover,energydepositsfromphysicalpar-
ticlestendtohavealargefractionofthepulseenergyinthepeakBX.Weuseaseriesofoffline
criteriathatexploitthesetimingandtopologicalcharacteristicstoremovespuriouseventsdue
to noise, which do not exhibit these properties. These criteria are detailed in Ref. [15] and are
appliedasinthatanalysiswiththeexceptionoftherequirementthattheleadingjethasatleast
60%ofitsenergycontainedinfewerthan6towers.
5 Signal efficiency
UsingthefirststageoftheMCsimulationdescribedinSection3,weestimatetheprobabilityof
anR-hadrontostopwithintheinstrumentedregionsofthedetector. Inparticular,weareinter-
ested in R-hadrons that stop in the barrel region of the calorimeter, since these are the regions
where we can observe the subsequent jet-like energy deposits from the decay products. We
exclude the endcap calorimeters since the signal-to-background ratio is less favorable in this
region. R-hadrons could also stop within the iron yokes interleaved with the muon detector
system, but we expect a negligible efficiency to detect the corresponding decays. The simula-
tiondemonstratesthatthestoppingprobabilityisapproximatelyconstantovertherangeofR-
hadronmassesconsideredinthissearch. TheprobabilitythatatleastoneR-hadronisstopped
within the barrel region of the calorimeter is found to be 8% for gluinos with m = 800GeV
g(cid:101)
and6%fortopsquarkswith m = 400GeV. Theslighterlarger ε obtainedforgluinosis
(cid:101)t stopping
becauseoftheirgreaterpropensitytoformdoublychargedstates.
For the particles that stop and then decay within the calorimeter, MC simulations estimate
5
o
ec CMS
εr
0.5 Simulation
0.4
0.3
m = 400 GeV
g~
m = 700 GeV
0.2 g~
m = 1200 GeV
g~
m~ = 300 GeV
t
m~ = 400 GeV
0.1 t
m~ = 600 GeV
t
m~ = 1000 GeV
t
0
50 100 150 200 250 300 350
E (E) [GeV]
g t
Figure1: Thereconstructionefficiencyεreco forg(cid:101)and(cid:101)tR-hadronsthatstopinthebarrelregion
ofthecalorimeterasafunctionoftheenergyoftheproducedSMdaughterparticle. Theshaded
bandsindicatethesystematicuncertaintyinε .
reco
an approximate trigger efficiency of 70%. We define ε as the number of signal events that
reco
pass all selection criteria (including the trigger requirement) divided by the number of signal
events that stop within the barrel region of the calorimeter. The reconstruction efficiency de-
pends principally on the energy of the visible daughter particle of the R-hadron decay, which
we denote by E (E) if the daughter is a gluon (top quark). The reconstruction efficiencies
g t
obtained for gluinos and top squarks are plotted as a function of this energy in Fig. 1. Above
theminimumenergythresholdfortheSMdecayproducts,whereε becomesapproximately
reco
constant, Eg > 120GeV (Et > 150GeV), we obtain εreco ≈ 45% (32%) for g(cid:101)((cid:101)t) decays. The top
squark efficiency is lower than the gluino efficiency primarily because of t → bµν decays that
yield less visible energy in the calorimeter and are rejected by the muon vetoes. When E is
t
belowmt,whichcanhappenincaseswhenthemasssplittingbetweenthe(cid:101)tandχ(cid:101)0issmall,the
topquarkisoffthemass-shell.
Thesignalefficiencyisgivenbytheproductofε andε .
stopping reco
6 Backgrounds
It is possible for halo muons to escape detection in the endcap muon system. Escaping detec-
tion is uncommon, but owing to the high rate of halo production in the 2012 data collection
period,theexpectedhalobackgroundisnon-negligible. Weestimatethehalovetoinefficiency
usinga“tag-and-probe”method[27]thatanalyzesahigh-puritysampleofhalomuonstode-
terminetheratesatwhichwerecordhitsonbothendsoftheendcapmuondetectors,compared
totherateatwhichweseeonlythe“incoming”or“outgoing”portionsofthehalomuontrack.
Becauseoftimingandtriggereffects,wemayonlyobservetheoutgoinglegofthehalomuon,
with the incoming leg recorded in a previous BX. When the reverse occurs, we see only the
6 7 Sourcesofsystematicuncertainties
incomingleg. Additionally,itispossibleforthemuontoloseallofitsenergywithintheCMS
detector before reaching the opposite side CSC chambers, which also results in an incoming-
only event. We classify these events as to whether the halo originates from the clockwise or
counterclockwisebeam,andbinthembytheirgeometriclocationintheendcapmuonsystem.
After integrating these distributions, we measure a halo veto inefficiency of 1×10−5. This in-
efficiency is multiplied by the number of halo events vetoed from the search sample yielding
anaveragehalobackgroundestimateof8.0±0.4events,wheretheuncertaintyintheestimate
isowingtothelimitedsizeofthedatasamplesused.
To determine the rate at which cosmic ray muons escape detection by the cosmic muon veto,
we generate a sample of 300 million simulated cosmic events using CMSCGEN [28], which is
basedontheairshowerprogramCORSIKA[29],andhasbeenvalidatedin[30]. Afterrequiring
asubstantialenergydepositinthecalorimeterandtheabsenceofanyhitsinthemuonendcap
systemthatwouldcausetheeventtobeclassifiedasahalomuon,weestimatetheinefficiency
of the cosmic muon veto by dividing the number of events that escape this veto by the total
number of events. The inefficiency obtained in this manner is roughly 0.5%. After multiplica-
tionbythenumberofcosmicraymuoneventsvetoedfromthesearchsample,thiscorresponds
to a predicted cosmic background of 5.2±2.5 events, where the uncertainty in the estimate is
owingtothelimitedsizeofthedatasamplesused.
Finally,thebackgroundowingtoinstrumentalnoiseisestimatedbyconsideringdatarecorded
in 2010. Figure 2 shows the measured noise rates in both periods. In these plots all selection
criteria except those that are designed to reject noise are applied. Additionally, only events
at least 5 BX from a bunch are considered. This is done to reduce halo contamination in the
distributions, which is abundant directly after a bunch crossing. There is a greater variation
in the 2012 noise rate because of increased halo background, which can mimic HCAL noise if
noCSChitsarepresent. Thesmallvariationseeninthe2012data, while largerthanthatseen
in2010,isneverthelesssmallcomparedtothesystematicuncertaintyinthenoiseeventcount.
The larger variations observed in the 2012 data are attributed to residual halo contamination
arisingfromnon-standardLHCbeamconditions.
Becausetherateofinstrumentalnoiseisapproximatelyconstantbetweenthetwoperiods,and
the data recorded in early 2010 were delivered with very low instantaneous luminosity, this
sample is unlikely to contain either halo events or signal events. After applying the same se-
lectioncriteriatothe2010datasampleasusedinthisanalysis,twoeventsremain. Weestimate
the cosmic ray muon contribution to this sample to be 4.8±3.6 events. Because the cosmic
raybackgroundestimateexceedsthenumberofobservedevents,weassumeacentralvalueof
zero events for the instrumental noise contribution to the 2010 sample, and allowing for Pois-
son fluctuations in both the noise and cosmic ray muon contributions, set a 68% confidence
level(CL)upperlimitonthiscontributionof2.3events. Thisestimateisthenscaledbythera-
tioofthe2012and2010livetimes,resultinginanexpectednoisecontributionof0.0+2.6 events
−0.0
inthe2012dataset.
7 Sources of systematic uncertainties
The model-independent results of the counting experiment described in this paper have rel-
atively few systematic uncertainties. There is a 2.5% uncertainty in the integrated luminos-
ity[31]. Thereisa13%uncertaintyinthereconstructionefficiencyresultingfromthepossibil-
itythatevenabovetheminimumvalueofE (E),thisefficiencyisnotcompletelyindependent
g t
of the energy of the daughter particle as is assumed. This uncertainty is determined by con-
sidering the difference between the individual values of ε in Fig. 1 and the average value
reco
7
z] 0.2
H
e [ 0.18 CMS 2010
e rat 0.16
ois 0.14
al n 0.12
ent 0.1
m
u 0.08
nstr 0.06
I
0.04
0.02
0 1134 1225 1251 1262 1283 1299
z] 0.2
H
e [ 0.18 CMS 2012
e rat 0.16
ois 0.14
al n 0.12
ent 0.1
m
u 0.08
nstr 0.06
I
0.04
0.02
0 2692 2870 2929 3033 3200 3279
Figure2: Thenoiseratesfrom2010data(top)and2012data(bottom).
for all points above the minimum value of E (E). The shaded bands in the figure depict this
g t
uncertainty. Becausetheenergydepositsresultinginreconstructedjetsarenottheresultofjets
originatingfromthecenterofthedetector(asisthecaseforjetsoriginatingfromppcollisions),
theyarenotnecessarilydirectedradially,andstandarduncertaintiesinthejetenergyscale(JES)
do not apply. Instead, we determine the JES uncertainty by referencing a study performed on
the HCAL during cosmic ray data taking in 2008 [32]. This study compares the reconstructed
energy deposits in the HCAL for simulated cosmic ray events and cosmic ray events in 2008
data. Thesecomparisonsleadtoanestimateduncertaintyof∼2%onthesimulation. Asimilar
study comparing data and simulation for dijets originating at the interaction point conducted
with 2012 data yielded an uncertainty of <2% for jets striking the HCAL barrel with angles
ofincidencefrom0to60degrees[33]. WhilethestudydemonstratesthattheHCALresponse
is well simulated with an uncertainty of about 1%, we take a conservative JES uncertainty of
3% to compensate for any effects of stopped particle decays that these studies cannot test be-
cause of the potentially large angles that could sometimes be expected in the signal decays.
This value for the JES uncertainty leads to an uncertainty in the search results of about 2% at
theminimumvalueof E . Thevalueof3%issomewhatpessimisticsincetheuncertaintyfalls
g
rapidly as E increases. Variations in the reconstructed jet energy are only important for de-
g
posits with energies close to the jet energy threshold, which typically correspond to events in
which E issmall.
g
In obtaining constraints on a particular model, however, more substantial uncertainties arise
since the signal yield is sensitive to the stopping probability. While the GEANT4 simulation
used to derive the stopping probability very accurately models both the electromagnetic and
nuclear interaction energy-loss mechanisms, the relative contributions of these energy-loss
mechanisms to the stopping probability depends significantly on unknown R-hadron spec-
8 8 Results
troscopy. We do not consider this dependence to be a source of error, however, since given a
particularmodelforthespectrumtheresultantuncertaintyinthestoppingprobabilityissmall.
Inadditiontotheseuncertaintiesinthesignalefficiency, thereisalsoasystematicuncertainty
inthebackgroundestimatedescribedinSection6. Thissystematicuncertaintyarisesfromthe
limited size of the data control samples that were used to estimate the contribution of each of
thebackgroundprocessestothesearchsample.
ThesystematicuncertaintiesaresummarizedinTable1.
Table1: Summaryofsystematicuncertainties.
Systematicuncertainty Fractionaluncertainty
JESuncertainty ±3%
Luminosityuncertainty ±2.5%
ε uncertainty ±13%
reco
Backgrounduncertainty +27%,−19%
8 Results
The total and individual background estimates for both the 2012 search period and the 2010
controlperiodusedtodeterminethebackgroundfrominstrumentalnoise,aresummarizedin
Table2,togetherwiththenumberofobservedevents.
Table 2: Background predictions and observed events for the 2010 control and 2012 search
samples.
Period Triggerlivetime(h) Nbkg Nbkg Nbkg Nbkg Nobs
noise cosmic halo total
2010 253 0.0+2.3 4.8±3.6 — 4.8+4.3 2
−0.0 −3.6
2012 281 0.0+2.6 5.2±2.5 8.0±0.4 13.2+3.6 10
−0.0 −2.5
With the assumption that the backgrounds listed in Table 2 are uniformly distributed in time,
whichisvalidevenforthehalobackgroundfortimesatleastoneBXawayfromthecollisionas
our selection requires, we perform a counting experiment in equally spaced log(time) bins of
gluino(topsquark)lifetimehypotheses,τ (τ),from10−7to106seconds. Forlifetimehypothe-
g(cid:101) (cid:101)t
sesshorterthanoneorbit(89µs),wecountonlycandidateswithinasensitivity-optimizedtime
windowof1.3τ (τ)fromanyppcollision. Thisrestrictionavoidstheadditionofbackgrounds
g(cid:101) (cid:101)t
for time intervals during which the signal has a high probability to have already decayed. In
ordertoresolveanytimestructureinthedatawithinasingleorbit,wetesttwoadditionallife-
time hypotheses for each observed event for these counting experiments: the largest lifetime
hypothesis for which the event lies outside 1.3τ (τ), and the smallest lifetime hypothesis for
g(cid:101) (cid:101)t
whichtheeventiscontainedwithin1.3τ (τ). Table3showstheresultsofthecountingexper-
g(cid:101) (cid:101)t
imentsforselectedlifetimehypotheses. Theobservednumberofeventsisconsistentwiththe
backgroundexpectationforalllifetimehypothesestested.
8.1 Limits on gluino and top squark production
WeobtainupperlimitsonthesignalproductioncrosssectionusingahybridCL method[34,
S
35] to incorporate the systematic uncertainties [36]. These limits are presented in Fig. 3 as a
function of particle lifetime τ. The two left-hand axes of Fig. 3 are production cross section
times branching fraction (σ×B) for top squarks and gluinos, assuming the total visible en-
ergy in the decay satisfies either E > 120GeV or E > 150GeV for the gluino and top squark
g t